1. Field of the Invention
The present invention relates to a filter media with at least two regions with distinct media properties that result in an enhanced sealing of filter media between individual layers or between the filter media and a sealing means. More specifically, the present invention relates to a surface property modification of filter media and/or a sealing surface to enhance the sealing of the filter media within various filter housings. More particularly, the present invention relates to a surface property modification which repels liquid at the points of contact between the porous filter material and the sealing surfaces of the filter housing. Most particularly, the present invention relates to an improved filter medium that results in an improved sealing of filters and provides means for fluid-gas transfer
2. Discussion of the Related Art
It is well known in the art that one of the most important aspects of a filter design, especially in critical fluid filter applications such as medical products and semi-conductor applications, is the prevention of filter failure.
In critical applications, even minor amounts of fluid bypass result in a nonconforming fluid for the application at hand. Therefore, providing good sealing between various components of a filter or filtration system is critical.
One of the most common, but not the most reliable, sealing mechanisms to date is a pinch seal. In a typical pinch seal the filter media is squeezed between two sealing surfaces. In a typical seal, special attention must be focused upon the amount of pressure imposed on the filter media to assure proper sealing and to minimize the possibility of fluid bypass. Special attention must be focused upon the structural properties of the media and its capability to handle the stress caused by the pinch seal. Typically, in a pinch seal, a designer would like to provide a maximum pressure at the seal without compromising the stability of the filter structure. The higher the pressure at the sealing surface, the higher the compression of the media at point of contact. Typically, this can result in a reduced pore size in the affected area, and also reduce the gap or pore size between the filter media and the sealing surface. This prevents fluid flow past the pinch seal as long as the pore size is small enough to block flow. Another way to stop flow past the seal is to block the pores proximate the seal with potting compound.
The present invention provides a better seal without the aforementioned problems. This is accomplished by making the seal material non-wetting to the fluid being filtered and, thereby, providing a liquid repellant seal. More specifically, a liquid repellant porous seal with a liquid wetting filter area. The wetability (fluid wetting properties) of the seal may be adjusted by methods known in the art. The present invention provides a differential wetability at the sealing surface.
The present invention teaches that the wetability of the seal may be decreased to prevent fluid bypass past the seal. At the same time, the main filter area is desired to be fluid wettable, especially for fluid filter applications, to provide fast priming times. In gravity feed systems or low pressure applications, the wetability of the filter medium is important as there is a minimum breakthrough pressure required to start fluid flow through a filter. The breakthrough pressure decreases with increased wetability. Therefore, the present invention provides a differential wetability of the filter media at the sealing surface. Also, it is noted that the surface property of the sealing surfaces, such as the housing, may be changed to enhance the seal.
In general, the wetability of a porous material depends on a number of parameters, including pore size and surface properties of the material involved. In addition, there will be a number of other advantages to the present invention that will be apparent from the descriptions provided and the applications shown.
Present day filter media, whether in the form of flat sheets of various shapes, such as circles, ovals, or other desired shapes, or in tubular form, are generally homogeneous throughout, and consist entirely of filter media without any treatment to the sealing regions to enhance sealing. This produces problems in the art. With filter media sheets, there tends to be leakage past the commonly used pinch seal, unless this seal is carefully designed. Such attention to the seal increases the cost of the filter housing holding the media, may result in additional components, and an increase in final product weight, to effect a proper seal. Sometimes, adhesives or other agents are needed to effect any seal at all. Also, since the pinch seal is typically wetted by the fluid, there is no possibility of a visual aide in discerning if the seal is adequate and the fluid being filtered is not bypassing the seal. A visual aide in detecting fluid bypass may provide an opportunity to save the fluid such that it may be processed in a single pass.
The same problems occur with tubular filters when attempting to provide effective seals at or near the ends of the filter tubes. The prior art is replete with special types of end caps and housing modifications designed to effectuate the seal between the tubular filter media, which is usually made of randomly oriented fibers (particles), and an end cap and/or housing. Again, these measures increase costs for the filter products. Thus, those skilled in the filter art continued to search for a better method of sealing filter media within filter housings.
In critical applications adhesive may be used to increase the reliability of the filter seal by typically filing the gaps or pores between the sealing surface and the media. The adhesive may be surface loaded in the area of the seal, or the adhesive may be present throughout the depth of the filter media for added protection against fluid bypass.
The addition of adhesive, typically provides a barrier to fluid bypass past the sealing surface. But in addition, it may provide added complications in certain applications. These include separation of various components of the adhesive, which may result in physical property changes in the seal such as bio-compatibility, thermal stability and chemical compatibility, decreased process time as the adhesive is typically thickened to control wicking of the adhesive into the filter media, and reduced versatility of the adhesive and/or the media as each application needs to be considered on a case by case basis to ensure a proper seal. Also, it is important for the adhesive to bond properly to the housing to provide an adequate seal. Delamination of the adhesive between the various components may result in fluid bypass.
If the adhesive is a multi-component, the various components of the adhesive may separate due to the capillary forces present in the filter media and may compromise the properties of the adhesive and the seal. Also, in certain instances this may result in bio-compatibility and/or leachability issues.
In addition, adhesives may have different physical properties than the filter media and the housing. Certain environmental conditions may result in a compromise of the seal. An example of such instance may be when the thermal expansion property of adhesives may not match that of the housing. Therefore, certain thermal fluctuations (such as those seen in typical medical applications requiring autoclaving) may result in a compromising of the seal. This is especially important in medical applications requiring autoclaved parts or heat or steam sterilization.
The present invention provides novel solutions and enhancements to these problems in the prior art. The present invention will describe a hydrophobic (hydrophobic in the context of this application is synonymous with liquid repellant) barrier that may be used in conjunction with the use of adhesives. The hydrophobic barrier will provide a restriction to the wicking of adhesive past a desired point. In addition, it provides a general means of treating media that may be used with various adhesives. Currently, the properties of the filter media and the adhesive are chosen such as to control the wicking of the adhesive. The present invention provides a novel concept, a barrier, such that these factors are not as important. The media may be used with a various adhesives, and one adhesive may be used with a variety of media. The barrier also provides a clear and distinct area where the adhesive will be present. Advantages include reduced variation in filter properties, such as flow rate, efficiency and capacity, as the usable filter area is clearly defined by the barrier.
The present invention also provides novel means to provide an enhanced sealing mechanism for filtration and separation applications. One novel concept provides a porous seal with enhanced seals to decrease possibility of bypass. The novel concept minimizes fluid hold-up volume within a filter and enables greater fluid recovery. It also decreases cost of the filtration system by the possibility of eliminating components such as adhesive, and also reduces the weight of the filter by providing a porous structure seal through elimination or minimization of adhesive usage. In special configurations to be described, the present invention provides an integral venting means within the filter that is an integral part of the sealing means.
Further, the present invention describes a novel concept that provides enhanced sealing of filter by providing a barrier to fluids. The barrier provides a means to prevent fluid contact across a pinch seal. In its' simplest form, the barrier provides a definite and clear barrier against fluid wicking or fluid migration. One of the advantages of this novel concept is that it prevents separation of multi-component adhesives. In addition, it broadens the selection criteria for filter media and/or adhesive. This is due to the fact that the barrier provides a broader range of adhesives that may be used with a given filter media structure. The influence of adhesive properties such as surface tension, viscosity, and gel time are minimized and, as such, provides a greater flexibility in the production of various filters. Also, the barrier enables combining a number of filters into a single filter housing. The single housing concept reduces the number of components and provides overall cost savings.
Providing multiple filters in a single housing with integral non-wetting barrier enables a number of novel concepts for filtration and separation systems. Fluids may be processed side by side. The barrier may be penetrated at elevated pressures providing unique separation and mixing applications. For example, the filter may be subjected to sufficient positive and negative pressure to drive fluid across the barrier. The pressure may be applied by various means known in the art such as centrifugation, infusion pumps, syringe, etc. A number of applications will be described in this application to demonstrate the novelty of this concept.
Other advantages of the present invention will be apparent from the description hereby provided.